1. Field of the Invention
The invention is in the field of laser rangefinders operating in the pulsed mode which emits a train of pulses having a fast rising edge or falling edge, or both, in which the two way times-of-travel is measured to provide the range of a target, and a broadened return signal provide a signature of the target for comparison with a known target.
2. Description of the Prior Art
Laser rangefinder generally operate in one of two modes. One mode may be a continuous wave (CW) operation where the relative phase between the transmitted optical signal and the reflected signal determine the roundtrip time between the target and the transmitter. The other method is to use a pulsed laser which emits a pulse with a fast rising edge or falling edge, or both. The time-of-flight is measured which provides the distance between the target and the transceiver.
Depending on the signal-to-noise ratio (SNR), the ranging accuracy in the pulse mode can be excellent for strong SNR, or poor when operating under low SNR. By measuring the time when the rising edge crosses the threshold set by the user, and by comparing that time with the time of transmission, a roundtrip time can be calculated, and hence, the distance to the target determined since the range is related to time of flight by this equation, R=ct, where c is speed of light and t is time of flight. The accuracy of the laser rangefinder is also degraded by the target depth. In a real scenario, the laser beam illuminates the entire target and since the target has depth, each scattering center, which is located at different depths of the target, contributes to the reflected signal at unique times. The resulting signal is broadened in time and hence contributes to the degradation in ranging accuracy. For a Nd:YAG laser the ranging accuracy for a single pulse for a flat target is limited by the rise time (10 ns), or 3 meters, under poor transmitting conditions. For a CO.sub.2 laser with a rising time of 40 ns, the minimum ranging accuracy is 12 meters. To obtain a target range resolved signature that is unique to a particular target and viewing angle, a short transmitted pulse is necessary to illuminate the target or otherwise the return signal would be integrated not only in cross range but in depth as well. Clearly, with a target depth of 5 to 10 meters, target range resolved signatures cannot yield target identification if the probing pulse integrates over 20 meters. What is needed, therefore, to reduce the time integration of the pulsewidth is a laser source with short pulsewidths, of 1 to 2 nanoseconds, with sufficient power to generate unique range resolved cross sections. Compact, mechanically stable and simple CO.sub.2 lasers do not exist at this moment that can produce 10.6 .mu.m laser pulses with 1 to 2 nanosecond pulsewidths. One device which can yield 1 to 3 nanosecond pulses is the mode-locked laser, but this device is undesirable because of its undesirably long cavity length.
The signal to noise (SNR) limitations of the range resolved cross sections is overcome by the present inventive device which uses a CO.sub.2 transverse electrode discharge at atmospheric pressure (TEA) laser that can yield up to 10 megawatts of power for a reasonable size laser and has its output chopped into short pulsewidths by a modulator.